(1) Field of the Invention
The present invention relates to a fuel storage device provided with a tank, to an aircraft provided with such a tank, and to a method of controlling such a tank.
More particularly, but not exclusively, the invention relates to an aircraft and in particular to an aircraft having a rotary wing. A rotary wing aircraft presents specific characteristics that lead to differences that are considerable compared with an airplane or a car.
(2) Description of Related Art
A fuel storage device usually comprises at least one tank containing the fuel. In addition, the storage device usually includes an expansion volume.
Such an expansion volume serves to receive some of the fuel stored in the tanks in the event of the fuel expanding. The expansion volume then avoids high pressures being applied to the walls of the tanks, and/or avoids fuel overflowing, e.g. via a vent system.
Aircraft certification regulations may require a minimum volume for this expansion volume. For example, paragraph 969 of the certification regulation known under the name “CS 29” specifies that the expansion volume must be greater than or equal to 2% of the total volume of tanks.
Furthermore, that paragraph specifies that it must be impossible to fill that expansion volume inadvertently when the aircraft is in a normal position on the ground.
It should be observed that the term “normal position” or “normal condition” designates positions that the manufacturer of the vehicle has authorized for use during filling. These authorized positions may be defined for example relative to threshold roll and pitching angles for the vehicle.
It may thus be strictly forbidden to fill the expansion volume even in part when filling the tanks, in order to ensure that the expansion volume is fully available in the event of the fuel expanding in flight.
Furthermore, a fuel storage device generally includes a vent circuit enabling some of the gas contained in the storage device to be discharged into the atmosphere.
In order to provide an expansion volume, a first technique makes provision for using the top portion of a tank as an expansion volume, this expansion volume communicating with the atmosphere via a vent system.
Nevertheless, it can be understood that the filler orifice needs to be positioned with care so that the top portion of the tank is not filled when filling the tank. This top portion must then be vertically above a filling plane that contains the filler orifice.
That first technique presents the advantage of being simple. However, the constraint associated with the positioning of the filling orifice can make it difficult to incorporate in certain aircraft, and in particular in aircraft presenting tanks arranged within a lower section of the fuselage arranged beneath a cabin floor.
In a second technique, an expansion vessel is used that defines the required expansion volume.
The expansion vessel is distinct from the tank with which it is in communication. Under such circumstances, the expansion vessel may be offset from the tank, e.g. in a high portion of a vent circuit.
It can be understood that it is then easier to position the expansion space in a plane situated above a filler orifice. Nevertheless, it can be difficult to find space within an aircraft that is suitable for receiving the expansion vessel.
Furthermore, this second technique sometimes leads to providing auxiliary safety means in order to avoid a risk of fire in the event of a crash, possibly made worse by the presence of expanded fuel in a remotely-located expansion vessel.
Finally, it will be understood that that second technique leads to a configuration that is relatively complex, and to difficulties in maintenance.
In the automobile field, fuel tanks are known that are hermetically closed by a filler cap that seals hermetically and that is provided with an auxiliary device for putting the content of the tank to atmospheric pressure.
That auxiliary device is obtained by a pipe going from the top portion of the tank towards a container containing activated carbon.
This container contains activated carbon in order to depollute the gas that has escaped from the fuel tank before it is exhausted into the atmosphere.
In addition, the system has an expansion vessel arranged between the fuel tank and said container.
In order to prevent fuel penetrating into the expansion vessel during filling, the car is generally provided with devices for closing the vent circuit while the filler cap and/or the filler hatch provided in the bodywork of the vehicle are open.
Rotary wing aircraft and in particular helicopters are conventionally not fitted with a container for a gas filter. The fuel tank is connected directly to the atmosphere by pipes, possibly via at least one roll-over valve in the vent serving to prevent the fuel tank being emptied through the vent circuit in the event of the aircraft overturning.
There is no need for the fuel to be subjected to any particular pressure on such a vehicle. That is why the person skilled in the art sometimes uses the first above-mentioned technique in order to obtain an expansion volume inside the tank.
On a vehicle of the airplane type, the fuel tanks are generally arranged in the wings. The configuration problem associated with an airplane is thus completely different from the problem associated with a rotary wing aircraft, e.g. of the helicopter type.
On a light airplane, fuel is generally refilled via a hatch arranged in the top wall of the fuel tank. Since the wing usually slopes, a fraction of the tank remains empty after filling and thus represents the required expansion volume.
In some airplanes, a plurality of tanks are arranged inside the wings. Those tanks communicate with a vent chamber via a vent line.
In the event of the fuel expanding in flight, the fuel may penetrate into the vent chamber. Transfer pumps then serve to return the fuel from the vent chamber back to the tank.
Document U.S. Pat. No. 7,621,483 describes a configuration of that type.
The tanks are under pressure. In addition, the fuel storage device presents complexity and dimensioning that appear not to be compatible with the requirements of rotary wing aircraft, in particular of the helicopter type.
In the technological background, there is document DE 199 25 728, which describes an expansion vessel arranged in series in a vent circuit. The expansion vessel is also arranged inside a tank.
Under such circumstances, the expansion volume defined by the expansion vessel communicates firstly with the inside of the tank via an assembly having at least a first valve, and secondly with the outside via a second valve and an activated carbon treatment system.
As a function of the pressure difference between the expansion volume and the inside of the tank, the fuel can move from the tank towards the expansion vessel, and vice versa.
Furthermore, document EP 0 233 681 describes a fuel storage device having a tank and an expansion vessel arranged inside the tank.
Document US 2005/199294 describes a tank connected to a filler tube opening out at a filler orifice. In addition, a vent line connects the tank to the outside atmosphere.
Under such circumstances, an expansion vessel is arranged outside the tank, the expansion vessel being connected by a high hose and a low hose to said vent line.
In addition, the filler orifice is also present in a plane below said hoses.
Document US 2002/121300 describes a tank connected to a filler tube leading to a filler orifice. In addition, an expansion vessel is arranged in the tank. A pipe then extends from said expansion vessel to a vapor treatment system.
Document DE 37 19 834 describes a tank. In addition, an expansion vessel is arranged in the tank. A pipe then extends from said expansion vessel to the outside.
Also known is Document DE 41 21 321. The expansion vessel has a wall made of foam with open cells, and the expansion vessel fills with fuel less quickly than the tank.
Although advantageous, that storage device does not appear to be compatible with the certification regulations that specify that it must not be possible for the expansion volume to be filled inadvertently when the aircraft is in a normal position on the ground.
Furthermore, depending on the shape of the tank and on the position of the feed zone, it is possible that there is a volume of fuel that cannot reach the feed pipe. This volume of fuel is sometimes referred to as the “non-consumable volume” insofar as this volume of fuel cannot be consumed by an engine.
In order to minimize the non-consumable volume in tanks, manufacturers use sloping surfaces in the bottoms of tanks so as to direct fuel towards the feed pipe. Such surfaces are referred to herein as “slopes” for simplification purposes. The slopes may slope in the longitudinal direction and in the transverse direction of the vehicle.
Under such circumstances, tanks may be made out of flexible material. By way of example, such a material comprises a polyester support with a mixture of elastomers. Foams inserted in the compartment of tanks thus make it possible to provide slopes on which a flexible tank rests.
Nevertheless, installing foams and selecting shapes for the bottom corners of tanks lead to non-negligible losses of the volume of fuel that can be taken on board. In addition, differences in performance between aircraft mean that the shape of each tank needs to be designed as a function of the aircraft, thus preventing any type of standardization.
Furthermore, the added foams present non-negligible weight.
Document U.S. Pat. No. 5,927,651 discloses a fuel tank device having a deformable elastomer structure capable of adapting to the shapes of a compartment.
Document FR 2 294 913 discloses expandable fuel storage means capable of being expanded and of occupying a position in which they are located at least in part outside a zone of an airplane referred to as a “surface zone”.
Document U.S. Pat. No. 3,409,253 discloses a retractable tank system for aircraft, the system being made up of rigid means surrounding a flexible surface for adapting to the quantity of fuel taken on board.
It can be understood that a fuel storage device constitutes a system that is complex and difficult to optimize. The consumable volume that is stored may be reduced by an expansion volume and by including slopes.
Documents EP 2 135 805, U.S. Pat. No. 2,736,356, U.S. Pat. No. 2,657,884, and U.S. Pat. No. 6,021,978 are remote from the field of the invention.
Document EP 2 135 805 describes an open-topped tank. A bag is arranged inside the tank, with the volume of the bag depending on the pressure that exists outside the tank.
Document U.S. Pat. No. 2,657,884 describes a tank having inflatable members arranged between stiffeners. Those inflatable members seek to remove fluid that becomes caught between the stiffeners.
Document U.S. Pat. No. 2,736,356 describes a fuel tank that is separated from pressurizing means by a movable diaphragm.
Document U.S. Pat. No. 6,021,978 describes a volume seeking to protect a tank against risks of explosion.